In an attempt to increase the efficiencies and performance of contemporary gas turbine engines, engineers have progressively pushed the engine environment to more extreme operating conditions. The harsh operating conditions of high temperature and pressure that are now frequently specified place increased demands on engine component-manufacturing technologies and new materials. Indeed the gradual improvement in engine design has come about in part due to the increased strength and durability of new materials that can withstand the operating conditions present in the modern gas turbine engine. With these changes in engine materials there has arisen a corresponding need to develop new repair and coating methods appropriate for such materials.
Turbine engines are used as the primary power source for many types of aircrafts. The engines are also auxiliary power sources that drive air compressors, hydraulic pumps, and industrial gas turbine (IGT) power generation. Further, the power from turbine engines is used for stationary power supplies such as backup electrical generators for hospitals and the like.
Most turbine engines generally follow the same basic power generation procedure. Compressed air generated by axial and/or radial compressors is mixed with fuel and burned, and the expanding hot combustion gases are directed against stationary turbine vanes in the engine. The vanes turn the high velocity gas flow partially sideways to impinge on the turbine blades mounted on a rotatable turbine disk. The force of the impinging gas causes the turbine disk to spin at high speed. Jet propulsion engines use the power created by the rotating turbine disk to draw more air into the engine, and the high velocity combustion gas is passed out of the gas turbine aft end to create forward thrust. Other engines use this power to turn one or more propellers, fans, electrical generators, or other devices.
The high pressure turbine (HPT) blade is one engine component that directly experiences severe engine conditions. Turbine blades are thus designed and manufactured to perform under repeated cycles of high stress and high temperature. An economic consequence of such a design criteria is that currently used turbine blades can be quite expensive. It is thus highly desirable to maintain turbine blades in service for as long as possible, and to return worn turbine blades to service, if possible, through acceptable repair procedures.
Turbine blades used in modern gas turbine engines are frequently castings from a class of materials known as superalloys. The superalloys include nickel-, cobalt- and iron-based alloys. In the cast form, turbine blades made from superalloys include many desirable elevated-temperature properties such as high strength and good environment resistance. Advantageously, the strength displayed by this material remains present even under stressful conditions, such as high temperature and high pressure, that are experienced during engine operation.
While the superalloys exhibit superior mechanical properties under high temperature and pressure conditions, they are subject to attack by chemical degradation. The gases at high temperature and pressure in the turbine engine can lead to hot corrosion and oxidation of the exposed superalloy substrates in turbine blades. Those turbine blades at the high pressure stages following the combustion stage of a gas turbine engine are particularly subject to this kind of erosion, and the portion of a turbine blade at the blade tip is even more subject to corrosion and oxidation as this area of the blade also experiences high pressure and temperature. Blade tips are also potential wear points. Corrosion and oxidation are both undesirable in that these processes can lead to the gradual erosion of blade tip material, which affects the dimensional characteristic of the blade as well as physical integrity. In order to protect superalloy turbine blades, a coating may be placed on both the airfoil surfaces, and the blade tip, to act as a barrier between the engine environment and the substrate material.
Among other materials, conventional MCrAlY coatings have been used as one kind of coating on turbine blades to resist corrosion and oxidation. In the conventional formulation of MCrAlY, M represents one of the metals Ni, Co, or Fe or alloys thereof. Cr, Al, and Y are the chemical symbols for Chromium, Aluminum, and Yttrium. Some conventional MCrAlY formulations are discussed in the following U.S. Pat. Nos. 4,532,191; 4,246,323; and 3,676,085. Families of MCrAlY compositions are built around the Nickel, Cobalt, or Iron constituents. Thus the literature speaks of NiCrAlY, NiCoCrAlY, CoCrAlY, CoNiCrAlY, and so on. Nevertheless there is a need to further improve MCrAlY formulations. It would be desired to develop modified MCrAlY formulations that impart improved corrosion and environmental resistance on engine components.
In conventional methods, MCrAlY is applied to a turbine blade as a coating layer through a thermal spray coating process like low pressure plasma spray (LPPS) or electron beam physical vapor deposition (EBPVD). In the thermal spray coating process the MCrAlY coating adheres to the surface of the substrate through mechanical bonding. The MCrAlY coating adheres to asperities previously fashioned onto the substrate surface. This process does not result in a metallurgical or chemical attachment of the MCrAlY material to the underlying substrate. This is described in U.S. Pat. No. 6,410,159. Other deposition techniques that have been used with MCrAlY coatings include CVD, EB/PVD, HVOF, and LPPS. Each of these coating approaches may require complex coating procedures. Additionally expensive equipment such as LPPS, EB/PVD, and sputtering may also be required to apply an overlay coating. Thus, a need exists to utilize a relatively low cost process for applying an MCrAlY coating, as compared to existing methods.
The option of throwing out worn turbine blades and replacing them with new ones is not an attractive alternative. The HPT blades are expensive. A turbine blade made of superalloy can be quite costly to replace, and a single stage in a gas turbine engine may contain several dozen such blades. Moreover, a typical gas turbine engine can have multiple rows or stages of turbine blades. Consequently there is a strong financial need to find an acceptable repair or coating method for superalloy turbine blades.
Hence, there is a need for a turbine engine component coating method that addresses one or more of the above-noted drawbacks. Namely, a method is needed that provides an improved MCrAlY protective layer over the component substrate, and/or a method that allows the efficient and economical deposition of MCrAlY onto a superalloy substrate and/or a modified MCrAlY composition that provides improved properties and durability, and/or a method that by virtue of the foregoing extends turbine blade life and is therefore less costly as compared to the alternative of replacing worn turbine parts with new ones. The present invention addresses one or more of these needs.